This invention relates to the ultrasonic testing of ceramic, metallic and other articles such as pipe, plates and the like using shear waves and more particularly to ultrasonic testing at high temperatures of about 300.degree.-1000.degree. C.
Conventional ultrasonic testing is based on longitudinal waves and mode converted shear waves. Testing by the use of normal incidence shear waves is particularly useful for measurements of thickness and certain physical characteristics such as shear modulus with good accuracy. In general, the velocity of the shear wave is less than that of the longitudinal wave and therefore provides a greater resolution and an improved accuracy. These tests are usually carried out at frequencies in the order of about 1-10 MHz and often about 2-5 MHz.
Previously, ultrasonic testing with shear waves have been carried out at low temperatures using a variety of couplants such as a wedge of plastic (for mode converting longitudinal waves to shear waves), viscous liquids, and adhesives such as epoxy.
Other couplants for ultrasonic testing include adhesives coated with or containing minute glass beads as disclosed in U.S. Pat. Nos. 3,678,737, 3,714,816 and 3,732,444; and certain polymeric materials such as silicones, polyurethanes and polyesters as disclosed in U.S. 3,663,842 and U.S. Pat. No. 3,763,694. These couplants appear to be directed to the use of longitudinal waves.
In particular with regard to couplants for ultrasonic testing by shear waves, the above couplants when used for nonpermanent attachment of the probe do not perform satisfactorily when the article is at temperatures in the order of 300.degree.-1000.degree. C. In general, the materials have limited stability (physically and/or chemically) at the higher temperatures. In addition, as the test conditions are changed between high and low temperatures, some solid couplants tend to break apart.
Accordingly, one object of the invention is a high temperature couplant for coupling a shear-wave transmitting probe to an article. Another object is a high temperature couplant which remains useful after being subjected to significant temperature variations. A further object is a technique for the ultrasonic testing of articles at high temperatures. These and other objects will become apparent from the following detailed description.